Immunoglobulins represent the most prevalent biopharmaceutical products in either manufacture or development worldwide. The high commercial demand for and hence value of this particular therapeutic market has led to the emphasis being placed on pharmaceutical companies to maximize the productivity of their respective mAb manufacturing processes whilst controlling the associated costs.
Affinity chromatography is used in most cases, as one of the key steps in the purification of these immunoglobulin molecules, such as monoclonal or polyclonal antibodies. A particularly interesting class of affinity reagents is proteins capable of specific binding to invariable parts of an immunoglobulin molecule, such interaction being independent on the antigen-binding specificity of the antibody. Such reagents can be widely used for affinity chromatography recovery of immunoglobulins from different samples such as but not limited to serum or plasma preparations or cell culture derived feed stocks. An example of such a protein is staphylococcal protein A, containing domains capable of binding to the Fc and Fab portions of IgG immunoglobulins from different species.
Staphylococcal protein A (SpA) based reagents have due to their high affinity and selectivity found a widespread use in the field of biotechnology, e.g. in affinity chromatography for capture and purification of antibodies as well as for detection. At present, SpA-based affinity medium probably is the most widely used affinity medium for isolation of monoclonal antibodies and their fragments from different samples including industrial feed stocks from cell cultures. Accordingly, various matrices comprising protein A-ligands are commercially available, for example, in the form of native protein A (e.g. Protein A SEPHAROSE™, GE Healthcare, Uppsala, Sweden) and also comprised of recombinant protein A (e.g. rProtein A SEPHAROSE™, GE Healthcare). More specifically, the genetic manipulation performed in the commercial recombinant protein A product is aimed at facilitating the attachment thereof to a support.
These applications, like other affinity chromatography applications, require comprehensive attention to definite removal of contaminants. Such contaminants can for example be non-eluted molecules adsorbed to the stationary phase or matrix in a chromatographic procedure, such as non-desired biomolecules or microorganisms, including for example proteins, carbohydrates, lipids, bacteria and viruses. The removal of such contaminants from the matrix is usually performed after a first elution of the desired product in order to regenerate the matrix before subsequent use. Such removal usually involves a procedure known as cleaning-in-place (CIP), wherein agents capable of eluting contaminants from the stationary phase are used. One such class of agents often used is alkaline solutions that are passed over said stationary phase. At present the most extensively used cleaning and sanitizing agent is NaOH, and the concentration thereof can range from 0.1 up to e.g. 1 M, depending on the degree and nature of contamination. This strategy is associated with exposing the matrix for pH-values above 13. For many affinity chromatography matrices containing proteinaceous affinity ligands such alkaline environment is a very harsh condition and consequently results in decreased capacities owing to instability of the ligand to the high pH involved.
An extensive research has therefore been focused on the development of engineered protein ligands that exhibit an improved capacity to withstand alkaline pH-values. For example, Gülich et al. (Susanne Gülich, Martin Linhult, Per-{dot over (A)}ke Nygren, Mathias Uhlén, Sophia Hober, Journal of Biotechnology 80 (2000), 169-178) suggested protein engineering to improve the stability properties of a Streptococcal albumin-binding domain (ABD) in alkaline environments. Gülich et al. created a mutant of ABD, wherein all the four asparagine residues have been replaced by leucine (one residue), aspartate (two residues) and lysine (one residue). Further, Gülich et al. report that their mutant exhibits a target protein binding behavior similar to that of the native protein, and that affinity columns containing the engineered ligand show higher binding capacities after repeated exposure to alkaline conditions than columns prepared using the parental non-engineered ligand. Thus, it is concluded therein that all four asparagine residues can be replaced without any significant effect on structure and function.
Recent work shows that changes can also be made to protein A (SpA) to effect similar properties. US patent application publication US 2005/0143566 discloses that when at least one asparagine residue is mutated to an amino acid other than glutamine or aspartic acid, the mutation confers an increased chemical stability at pH-values of up to about 13-14 compared to the parental SpA, such as the B-domain of SpA, or Protein Z, a synthetic construct derived from the B-domain of SpA (U.S. Pat. No. 5,143,844). The authors show that when these mutated proteins are used as affinity ligands, the separation media as expected can better withstand cleaning procedures using alkaline agents. US 2006/0194955 shows that the mutated ligands can better withstand proteases thus reducing ligand leakage in the separation process. Another publication, US 2006/0194950 shows that the alkali stable SpA domains can be further modified such that the ligands lacks affinity for Fab but retains Fc affinity, for example by a G29A mutation.
In order to decrease the replacement cost of affinity chromatography media, there is a strong desire from the bioprocessing industry to increase the number of cycles an affinity column can be used. This means that stability towards a large number of separation cycles, each involving treatment with 0.1-1 M sodium hydroxide, is a highly desirable property for a protein A product. Hence, there is still a need to further improve the alkali stability of ligands based on protein A domains or mutations of protein A domains.
Historically the native protein A containing 5 IgG binding domains was used for production of all protein A affinity media. Using recombinant technology a number of protein A constructs have been produced all containing 4 or 5 IgG binding domains. A recent study showed that dimeric ligands have a similar, or increased binding capacity compared to tetrameric ligands (WO 2010/080065). CN101337986 discloses a specific hexamer of Protein A B-domains with A1V and A1L mutations. The binding capacity of this hexamer is however low (in the order of 30 mg/ml).
There is still a need in this field to obtain a separation matrix containing protein ligands having an increased binding capacity.